Vanillin is obtained from natural sources such as lignin or ferulic acid, but a large proportion of vanillin is produced chemically.
Many diverse and varied preparation methods are described in the literature (Kirk-Othmer—Encyclopedia, of Chemical Technology 24, pp. 812-825, 4th edition (1997)).
A standard route of access to vanillin involves a condensation reaction of glyoxylic acid on gaiacol in basic medium, to obtain 4-hydroxy-3-methoxymandelic acid. This product is then oxidized to give vanillin.
The yield for the condensation is limited by the fact that the condensation reaction is not selective and also leads to o-hydroxymandelic acid and to dimandelic acids.
The formation, of dimandelic acids results from a subsequent reaction, namely a second condensation of glyoxylic acid with a mandelic acid.
It is sought to limit the subsequent reactions in order to obtain optimal selectivity.
Continuous processes are usually performed in a cascade of several reactors of perfectly stirred type.
However, multiplying the reactors induces costs both in the implementation and in the exploitation.